NXX Code. Central Office Code or CO Code The three digit switch entity indicator which is defined by the "D", "E", and "F" digits of a 10-digit telephone number within the NANP. Each NXX Code contains 10,000 station numbers.
NXX Code. The second three-digit sequence of each 10-digit telephone number within the NANP (i.e., NPA-NXX-XXXX).